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The light absorption capacity of water‐soluble humic‐like substances (HULISWS) at the molecular level is crucial for reducing the uncertainties in modeling the radiative forcing. This study proposed a machine learning approach to allocate the light absorption coefficient at 365 nm (Abs365) of HULISWS into 8084 Fourier transform‐ion cyclotron resonance mass spectrometry (FT‐ICR‐MS) detached molecular markers and their potential functional groups. The ML model showed an acceptable uncertainty (<5%) to the whole Abs365 value based on the prediction errors. The results showed that five critical light‐absorbing molecules (C4H6O4NS, C8H6O4NS, C11H15O3N2, C12H15O3N2, and C19H21O6) could explain 74% (±3%) of the variation of Abs365 in the winter, whereas no crucial light‐absorbing molecules were found in the summer. Besides, the nitrogen‐containing functional groups were found to dominate (61% ± 8%) the molecular absorption near the 365 nm of the spectrum. This work illustrated how functional groups affect the absorption of HULISWS, providing critical information for future research of HULISWS on the molecular level. Plain Language Summary: Water‐soluble humic‐like substances are a crucial light‐absorbing component of fine particulate matter in China. Understanding the crucial light‐absorbing chromophores of the light‐absorbing mixtures was helpful in controlling the global warming event. In this study, CHON and CHONS species were found to dominate the light absorption coefficient of water‐soluble humic‐like substances in Nanjing by combining the machine learning algorithm with the FT‐ICR‐MS technique. Then, the dominant light‐absorbing molecules were further combined with the UV‐Vis spectra from the chemical database to further estimate the light‐absorbing functional groups. Our results indicated that nitrogen‐containing functional groups dominate the light‐absorbing ability of water‐soluble humic‐like substances. Key Points: Appropriate synthetic minority over‐sampling based few‐shot learning could decipher the light‐absorbing molecules from a small data setC4H6O4NS, C8H6O4NS, C11H15O3N2, C12H15O3N2, and C19H21O6 dominate the Abs365 of HULISWS in NanjingNitrogen‐containing functional groups dominate the light‐absorbing ability of HULISWS at a micro‐level [ABSTRACT FROM AUTHOR]

Excessive reactive oxygen species (ROS) in the human body is an important factor leading to diseases. Therefore, research on the content of reactive oxygen species in atmospheric particles is necessary. In recent years, the online detection technology of ROS has been developed. However, there are few technical studies on online detection of ROS based on the dithiothreitol (DTT) method. Here, to modify the instrument, a DTT experimental module is added that is protected from light and filled with nitrogen at the end based on the Monitor for AeRosols and Gases in ambient Air (MARGA). The experimental study found that the detection limit of the modified instrument is 0.024 nmol min -1. The DTT consumption rate of blank sample (ultrapure water) is reduced by 44 %, which eliminates the influence of outside air and light in the experiment. And the accuracy of the online instrument is determined by comparing the online and offline levels of the samples, which yielded good consistency (slope 0.97, R2=0.95). It shows that the performance of the instrument is indeed optimized, the instrument is stable, and the characterization of ROS is accurate. The instrument not only realizes online detection conveniently and quickly, but also achieves the hour-by-hour detection of ROS based on the DTT method. Meanwhile, reactive oxygen and inorganic ions in atmospheric particles are quantified using the online technique in the northern suburbs of Nanjing. It is found that the content of ROS during the day is higher than that at night, especially after it rains; ROS peaks appear in the two time periods of 08:00–10:00 and 16:00–18:00 (China Standard Time, UTC + 8:00; expressed here in local time – LT). In addition, examination of the online ROS, water-soluble ion (SO42- , NO3- , NH4+ , Na+ , Ca2+ , K+), BC, and polluting gas (SO2 , CO, O3 , NO, NOx) measurements revealed that photo-oxidation and secondary formation processes could be important sources of aerosol ROS. This breakthrough enables the quantitative assessment of atmospheric particulate matter ROS at the diurnal scale, providing an effective tool to study sources and environmental impacts of ROS. [ABSTRACT FROM AUTHOR]

Inorganic nitrate production is critical in atmospheric chemistry that reflects the oxidation capacity and the acidity of the atmosphere. Here we use the oxygen anomaly of nitrate (Δ17O( NO 3 − )) in high-time-resolved (3 h) aerosols to explore the chemical mechanisms of nitrate evolution in fine particles during the winter in Nanjing, a megacity of China. The continuous Δ17O( NO 3 − ) observation suggested the dominance of nocturnal chemistry (NO3 + HC/H2O and N2O5 + H2O/Cl−) in nitrate formation in the wintertime. Significant diurnal variations of nitrate formation pathways were found. The contribution of nocturnal chemistry increased at night and peaked (72%) at midnight. Particularly, nocturnal pathways became more important for the formation of nitrate in the process of air pollution aggravation. In contrast, the contribution of daytime chemistry (NO2 + OH/H2O) increased with the sunrise and showed a highest fraction (48%) around noon. The hydrolysis of N2O5 on particle surfaces played an important role in the daytime nitrate production on haze days. In addition, the reaction of NO2 with OH radicals was found to dominate the nitrate production after nitrate chemistry was reset by the precipitation events. These results suggest the importance of high-time-resolved observations of Δ17O( NO 3 − ) for exploring dynamic variations in reactive nitrogen chemistry. [ABSTRACT FROM AUTHOR]

Excessive reactive oxygen species (ROS) in the human body is an important factor leading to diseases. Therefore, research on the content of reactive oxygen species in atmospheric particles is necessary. In order to more conveniently and accurately detect the content of reactive oxygen in atmospheric particles hour by hour. Here, to modify the instrument, it is added a DTT experimental module that is protected from light and filled with nitrogen at the end, based on the Monitor for AeRosols and Gases in ambient Air (MARGA). The experimental study found that the detection limit of the modified instrument is 0.024 nmol min-1. And the accuracy of the online instrument is determined by comparing the online and offline levels of the samples, which yielded good consistency (slope 0.97, R2 = 0.95). It shows that the performance of the instrument is indeed optimized, the instrument is stable, and the characterization of ROS is accurate. Meanwhile, reactive oxygen and inorganic ions in atmospheric particles are quantified using the online technique in the northern suburbs of Nanjing. It is found that the content of ROS during the day is higher than that at night, especially after it rains, ROS peaks appear in the two time periods of 08:00-10:00 and 16:00-18:00. In addition, examination of the online ROS and water-soluble ions (SO42-, NO3-, NH4+, Na+, Ca2+, K+), BC and polluting gases (SO2, CO, O3, NO, NOx) measurements revealed that photo-oxidation and secondary formation processes could be important sources of aerosol ROS. This method breakthrough enables the quantitative assessment of atmospheric particulate matter ROS at the diurnal scale, providing an effective tool to study sources and environmental impacts of ROS. [ABSTRACT FROM AUTHOR]

Volatile organic compounds (VOCs) are key components of tropospheric chemistry. We investigated ambient VOCs in an industrial area in Nanjing, China, between July 2018 and May 2020. The sum of the suite of measured total VOC (TVOC) concentrations was 59.8 ± 28.6 ppbv (part per billion by volume) during the investigation period. About twice the TVOC concentrations were observed in the autumn (83 ± 20 ppbv) and winter (77.5 ± 16.8 ppbv) seasons compared to those in spring (39.6 ± 13.1 ppbv) and summer (38.8 ± 10.2 ppbv). In previous studies in Nanjing, oxygenated VOCs (OVOCs) and halocarbons were not measured, and the current TVOC concentration without halocarbons and OVOCs was similar to the previous investigation in the same study area. However, it was twofold higher than the one reported in the nonindustrial suburban area of Nanjing. Due to the industrial influence, the halocarbons VOC group (14.3 ± 7.3 ppbv, 24 %) was the second-largest contributor to the TVOCs after alkanes (21 ± 7 ppbv, 35 %), which is in contrast with the previous studies in Nanjing and also in almost all other regions in China. Relatively high proportions of halocarbons and aromatics were observed in autumn (25.7 % and 19.3 %, respectively) and winter (25.8 % and 17.6 %, respectively) compared to those in summer (20.4 % and 11.8 %, respectively) and spring (20.3 % and 13.6 %, respectively). According to the potential source contribution function (PSCF), short-distance transport from the surrounding industrial areas and cities was the main reason for the high VOC concentrations in the study area. According to positive matrix factorization (PMF) model results, vehicle-related emissions (33 %–48 %) contributed to the major portion of the ambient VOC concentrations. Aromatics, followed by alkenes, were the top contributors to the loss rate of OH radicals (LOH ; 37 % and 32 %, respectively). According to the empirical kinetic modelling approach (EKMA) and relative incremental reactivity (RIR) analysis, the study area was in the VOC-sensitive regime for ozone (O 3) formation during all measurement seasons. Therefore, alkenes and aromatics emissions from automobiles need to be decreased to reduce secondary air pollution formation in the study area. [ABSTRACT FROM AUTHOR]

Carbonaceous aerosols have great influence on the air quality, human health and climate change. Except for organic carbon (OC) and elemental carbon (EC), brown carbon (BrC) mainly originates from biomass burning as a group of OC, with strong absorption from the visible to near-ultraviolet wavelengths, and makes a considerable contribution to global warming. Large numbers of studies have reported long-term observation of OC and EC concentrations throughout the world, but studies of BrC based on long-term observations are rather limited. In this study, we established a two-wavelength method (658 and 405 nm) applied in the Sunset thermal–optical carbon analyzer. Based on a 1-year observation, we firstly investigated the characteristics, meteorological impact and transport process of OC and EC. Since BrC absorbs light at 405 nm more effectively than 658 nm, we defined the enhanced concentrations (dEC = EC 405nm - EC 658nm) and gave the possibility of providing an indicator of BrC. The receptor model and MODIS fire information were used to identify the presence of BrC aerosols. Our results showed that the carbonaceous aerosol concentrations were the highest in winter and lowest in summer. Traffic emission was an important source of carbonaceous aerosols in Nanjing. Receptor model results showed that strong local emissions were found for OC and EC; however, dEC was significantly affected by regional or long-range transport. The dEC/OC and OC/EC ratios showed similar diurnal patterns, and the dEC/OC increased when the OC/EC ratios increased, indicating strong secondary sources or biomass burning contributions to dEC. A total of two biomass burning events both in summer and winter were analyzed, and the results showed that the dEC concentrations were obviously higher on biomass burning days; however, no similar levels of the OC and EC concentrations were found both in biomass burning days and normal days in summer, suggesting that biomass burning emissions made a great contribution to dEC, and the sources of OC and EC were more complicated. Large number of open fire counts from the northwestern and southwestern areas of the study site were observed in winter and significantly contributed to OC, EC and dEC. In addition, the nearby Yangtze River Delta area was one of the main potential source areas of dEC, suggesting that anthropogenic emissions could also be important sources of dEC. The results proved that dEC can be an indicator of BrC on biomass burning days. Our modified two-wavelength instrument provided more information than the traditional single-wavelength thermal–optical carbon analyzer and gave a new idea about the measurement of BrC; the application of dEC data needs to be further investigated. [ABSTRACT FROM AUTHOR]

Biomass burning can significantly impact the chemical and optical properties of carbonaceous aerosols. Here, the biomass burning impacts were studied during wintertime in a megacity of Nanjing, eastern China. The high abundance of biomass burning tracers such as levoglucosan (lev), mannosan (man), galactosan (gal) and non-sea-salt potassium (nss- K+) was found during the studied period with the concentration ranges of 22.4–1476 ng m -3 , 2.1–56.2 ng m -3 , 1.4–32.2 ng m -3 and 0.2–3.8 µ g m -3 , respectively. The significant contribution of biomass burning to water-soluble organic carbon (WSOC; 22.3±9.9 %) and organic carbon (OC; 20.9±9.3 %) was observed in this study. Backward air mass origin analysis, potential emission sensitivity of elemental carbon (EC) and MODIS fire spot information indicated that the elevations of the carbonaceous aerosols were due to the transported biomass-burning aerosols from southeastern China. The characteristic mass ratio maps of lev/man and lev/nss - K+ suggested that the biomass fuels were mainly crop residuals. Furthermore, the strong correlation (p<0.01) between biomass burning tracers (such as lev) and light absorption coefficient (babs) for water-soluble brown carbon (BrC) revealed that biomass burning emissions played a significant role in the light-absorption properties of carbonaceous aerosols. The solar energy absorption due to water-soluble brown carbon and EC was estimated by a calculation based on measured light-absorbing parameters and a simulation based on a radiative transfer model (RRTMG_SW). The solar energy absorption of water-soluble BrC in short wavelengths (300–400 nm) was 0.8±0.4 (0.2–2.3) W m -2 (figures in parentheses represent the variation range of each parameter) from the calculation and 1.2±0.5 (0.3–1.9) W m -2 from the RRTMG_SW model. The absorption capacity of water-soluble BrC accounted for about 20 %–30 % of the total absorption of EC aerosols. The solar energy absorption of water-soluble BrC due to biomass burning was estimated as 0.2±0.1 (0.0–0.9) W m -2 , considering the biomass burning contribution to carbonaceous aerosols. Potential source contribution function model simulations showed that the solar energy absorption induced by water-soluble BrC and EC aerosols was mostly due to the regionally transported carbonaceous aerosols from source regions such as southeastern China. Our results illustrate the importance of the absorbing water-soluble brown carbon aerosols in trapping additional solar energy in the low-level atmosphere, heating the surface and inhibiting the energy from escaping the atmosphere. [ABSTRACT FROM AUTHOR]

Water-soluble organic carbon (WSOC) is a significant fraction of organic carbon (OC) in atmospheric aerosols. WSOC is of great interest due to its significant effects on atmospheric chemistry, the Earth's climate and human health. The stable carbon isotope (δ13C) can be used to track the potential sources and investigate atmospheric processes of organic aerosols. However, the previous methods measuring the δ13C values of WSOC in ambient aerosols require a large amount of carbon content, are time-consuming and require labor-intensive preprocessing. In this study, a method of simultaneously measuring the mass concentration and the δ13C values of WSOC from aerosol samples is established by coupling the GasBench II preparation device with isotopic ratio mass spectrometry. The precision and accuracy of isotope determination is better than 0.17 ‰ and 0.5 ‰, respectively, for samples containing WSOC amounts larger than 5 µg. This method is then applied for the aerosol samples collected every 3 h during a severe wintertime haze period in Nanjing, eastern China. The WSOC values vary between 3 and 32 µg m -3 , whereas δ13C-WSOC ranges from -26.24 ‰ to -23.35 ‰. Three different episodes (Episode 1, Episode 2 and Episode 3) are identified in the sampling period, showing a different tendency of δ13C-WSOC with the accumulation process of WSOC aerosols. The increases in both the WSOC mass concentrations and the δ13C-WSOC values in Episode 1 indicate that WSOC is subject to a substantial photochemical aging during the air mass transport. In Episode 2, the decline of the δ13C-WSOC is accompanied by the increase in the WSOC mass concentrations, which is associated with regional-transported biomass burning emissions. In Episode 3, heavier isotope (13C) is exclusively enriched in total carbon (TC) in comparison to WSOC aerosols. This suggests that the non-WSOC fraction in total carbon may contain 13C -enriched components such as dust carbonate, which is supported by the enhanced Ca2+ concentrations and air mass trajectory analysis. The present study provides a novel method to determine the stable carbon isotope composition of WSOC, and it offers a great potential to better understand the source emission, the atmospheric aging and the secondary production of water-soluble organic aerosols. [ABSTRACT FROM AUTHOR]

Fine particulate matter samples (PM 2.5) were collected from the three megacities of the Yangtze River Delta (i.e., Shanghai, Hangzhou, and Nanjing) during November 11 to December 6, 2016. Organic carbon (OC), elemental carbon (EC), and four types of non-polar organic compounds such as polycyclic aromatic hydrocarbons (PAHs), n -alkanes, hopanes and steranes were analyzed in the samples. The average PM 2.5 mass concentrations were 47.0, 64.2 and 45.5 μg m−3 in Shanghai, Hangzhou and Nanjing, respectively. OC and EC concentrations were 4.47 and 0.78 μg m−3, 15.38 and 2.97 μg m−3, 10.35 and 2.06 μg m−3, respectively. n -Alkanes accounted for the largest abundance of the identified non-polar organic compounds, followed by PAHs, hopanes and steranes. The positive matrix factorization (PMF) analysis demonstrated that vehicular emissions, coal burning and industrial emissions were the most important sources. Vehicle exhaust was the major source of emissions in Shanghai (61.0%) and Hangzhou (45.3%), whereas coal combustion emissions was dominant (48.8%) in Nanjing. In addition, the estimated lifetime cancer risk (ILCR) values exposed to PAHs for adults (6.54 × 10−5) and children (7.32 × 10−6) suggested that PAHs levels in this region pose potential health risks. Combining the health risk model with PMF further concluded that vehicular emissions (49.0%) and coal burning (35.7%) were the two major sources that contribute to the ILCR values in these three cities. • Characteristics and source apportionment of NPOCs from three megacities in the YRD were studied. • NPOCs with unique sources were used to track the source of the aerosols. • Vehicular emissions and coal burning were most important contributors of NPOCs. • Polycyclic aromatic hydrocarbons pose a greater health risk to adult than children. • Combining the health risk model and PMF to reveal the contribution of each source to health risk. [ABSTRACT FROM AUTHOR]

Nitrate is the major component of fine particle matter in recent years, and fog deposition is an important sink form of atmospheric nitrate aerosol. However, evidence shows that high relative humidity, and stable atmosphere condition may lead to the rapid increase of secondary aerosols, including nitrate. Therefore, whether fog event is a sink or a source of nitrate aerosol is still an open question. Simultaneous observations of concentration and isotopes of nitrate both in PM 2.5 and fog water during fog event were conducted during a heavy fog event happened in winter of 2018 in Nanjing, a megacity sited in the Yangzi River Delta, China. Based on nine 12-h time resolution fog water samples and eleven 24-h time resolution PM 2.5 samples, it was found that NO 3 − concentration in PM 2.5 increased during the fog event (from 10.4 ± 8.6 μg m−3 to 21.2 ± 11.0 μg m−3). Compared with Δ17O–NO 3 - in PM 2.5 before (28.4 ± 3.7‰) and after the fog event (28.6 ± 1.3‰), Δ17O–NO 3 - in PM 2.5 during the fog event was higher (31.6 ± 1.2‰) and Δ17O–NO 3 - in fog water was lower (24.3 ± 1.2‰). The increased NO 3 − in PM 2.5 during the fog event had a different formation pathway compared with NO 3 − in PM 2.5 when there is no fog, and NO 3 − in fog water had another formation pathway as well. Explosive growth of NO 3 − in PM 2.5 may come from active NO 3 + HC reaction, along with happen of fog event through the reaction of NO 2 with ·OH or H 2 O and hydrolysis of N 2 O 5. As the fog event finished, the fog water possibly mainly settled NO 3 − newly generated during the fog event. It seems the fog event may be a source rather than a sink of atmospheric nitrate aerosols in this case. [Display omitted] • Nitrate is the main ion in fog water in urban area, mainly come from reaction of NO 2 with ·OH/H 2 O and hydrolysis of N 2 O 5. • Plenty of nitrate particles generated by reaction of NO 3 with HC/H 2 O during the fog event, mainly came from coal burning. • Fog water mainly settled nitrate particles newly generated during the fog event rather than that before the fog event. [ABSTRACT FROM AUTHOR]

Humic-like substances (HULIS), as important components of brown carbon (BrC), play an important role in climate change. In this study, one-year PM 2.5 samples from 2017 to 2018 were collected at Nanjing, China and the water soluble HULIS and other chemical species were analyzed to investigate the seasonal variations, optical properties and possible sources. The HULIS concentrations exhibited highest in winter and lowest in summer. The annual averaged HULIS concentration was 2.61 ± 1.79 μg m−3, accounting for 45 ± 13% of water-soluble organic carbon (WSOC). The HULIS light absorption coefficient at 365 nm (Abs 365, HULIS) averagely accounted for 71 ± 19% of that of WSOC, suggesting that HULIS are the main light-absorbing components in WSOC. The annual averaged Ångström absorption exponent and mass absorption efficiency of HULIS at 365 nm were 5.22 ± 0.77 and 1.71 ± 0.70 m2 g−1. Good correlations between HULIS with levoglucosan and K+ suggested biomass burning (BB) influence on HULIS. High concentrations of HULIS and secondary species (e.g., NO 3 −, SO 4 2−, NH 4 +, C 2 O 4 2−) were found in present of high relative humidity, indicating strong aqueous phase secondary HULIS formation. Secondary HULIS produced from anthropogenic and biogenic precursors were quantified based on the positive matrix factorization (PMF) model and the results showed that both fossil (55%) and biogenic (45%) emission sources made great contributions to HULIS. Fossil fuel combustion significantly contributed to HULIS formation throughout the whole year, which were enriched with more secondary HULIS (30%) than primary HULIS (25%). Strongest BB contribution (39%) was found in winter and biogenic SOA contribution (32%) was found in summer. A multiple linear regression (MLR) method was further applied to obtain specific source contributions to Abs 365, HULIS and the results showed that strong light-absorbing chromophores were produced from anthropogenic precursors. Our results highlight the anthropogenic SOA and fossil fuels combustion contributions to HULIS in addition to the biggest contributor, BB, in urban area in China. • Optical properties and sources of HULIS in PM 2.5 at Nanjing, China were explored. • High HULIS and secondary species levels were observed under high relative humidity. • Secondary HULIS from anthropogenic and biogenic precursors were quantified. • Both fossil (55%) and biogenic (45%) sources made great contributions to HULIS. • HULIS produced from anthropogenic precursors showed stronger UV absorption. [ABSTRACT FROM AUTHOR]

From 2013, a significant decline in fine particulate matter (PM 2.5) occurred in most Chinese cities. In Nanjing, a capital city in the central of the Yangtze River Delta (YRD) region, the mass concentrations of total carbon (TC) in PM 2.5 decreased by approximately 48% during wintertime from 2014 to 2019. To provide detailed information in this trend, source apportionment of TC was carried out using a radiocarbon-(14C-) based method. In general, the average mass concentrations (and fossil fuel proportions) of TC, organic carbon (OC) and elemental carbon (EC) in six winters were 16.3 ± 8.5 μg m−3 (51%), 14.4 ± 7.8 μg m−3 (48%) and 2.0 ± 1.2 μg m−3 (65%), respectively. From 2014 to 2019, the reduction in TC was mainly attributed to the decline in non-fossil sources (6.4, 6.3–6.5 μg m−3) rather than fossil-fuel sources (3.6, 3.4–3.7 μg m−3) (medians, 10th-90th percentiles). The reduction of OC (9.1 μg m−3) was mainly attributed to OC from biomass burning (5.3, 4.6–6.1 μg m−3) and secondary formation from fossil-fuel sources (3.3, 2.6–3.9 μg m−3). Although the inter-annual variations for EC were less obvious, the reduction was dominated by the variations in non-fossil emissions (i.e., biomass burning). This study highlights the important role of non-fossil source control in the reduction of TC mass concentration in the YRD. Strategies of future emission control may pay more attention to both biomass burning emissions and secondary organic aerosols from precursors originating from fossil sources. [Display omitted] •Source apportionment was carried out using a14C-based method for six winters. •Nearly 2/3 of the declined TC was attributed to the decrease of non-fossil emissions. •Reduction of SOC ff should be considered more in the future. [ABSTRACT FROM AUTHOR]

Atmospheric volatile organic compounds (VOCs) are not only harmful to human health, but also lead to ozone (O 3) formation. From July 3 to August 1 of 2018, online measurements of atmospheric VOCs were conducted in Nanjing City, in order to investigate the source apportionments to VOCs since the Empirical Kinetic Modelling Approach (EKMA) suggested that O 3 formation was VOC-limited at the receptor site. Using positive matrix factorization (PMF) model, we quantified eight sources of VOCs, including vehicle exhausts (23%), industrial source (18%), fuel evaporation (17%), petrochemical industry (12%), solvent usage (12%), biogenic emission (8%) and liquefied petroleum gas (7%) along with gasoline additive (3%). The diurnal distributions showed that the contributions of traffic-related sources maximized during the traffic rush hours. In contrast, biogenic sources had the highest contribution at noontime. Backward trajectory results showed that local traffic emissions were the main sources of VOC in Nanjing. Our results revealed that strict control of VOC emissions from local vehicle exhaust might be an important way to decrease high VOC pollution in Nanjing. • O 3 formation in Nanjing during the summertime was VOC-limited. • Vehicle exhausts and industries were the major sources to atmospheric VOCs. • Control of local traffic emissions was a better way to decrease VOC concentrations. [ABSTRACT FROM AUTHOR]

Hourly concentrations of 89 volatile organic compounds (VOCs) together with other atmospheric trace gases like ozone (O 3), oxides of nitrogen (NOx), carbon monoxide (CO), and sulfur dioxide (SO 2) were measured continuously in a suburban area of Nanjing, China. The investigations were conducted during the summer, 2018 to better characterize airborne VOC and their influence on O 3 and secondary organic aerosols (SOA) formation. The average hourly total VOCs (TVOCs) concentration was 35 ± 21 ppbv which was mainly contributed by different alkanes (41%) followed by halohydrocarbons and oxygenated volatile organic compounds (31%), aromatics (16%), alkenes (9%), and alkyne (3%). The TVOCs concentration was in a similar range with the ones observed in other urban and suburban areas in China. Traffic had an important influence on the air quality in the study area as the diurnal variation of the trace gases depicted a bimodal distribution that coincides with the rush-hours. The O 3 concentrations exceeded both the national and international air quality standards. The VOC:NOx was much higher than 8:1, indicating ambient air was NOx limited to atmospheric O 3 formation, therefore, reduction of NOx concentration could reduce O 3 formation rates more effectively. The average hourly ozone formation potential (OFP) of the VOCs was 218 μg m−3 and the major contributors to it were aromatics (43%) and alkenes (23%). The average hourly secondary organic aerosol formation potential (SOAFP) of the VOCs was 0.9 μg m−3. Similar to the OFP, aromatic VOCs were the major contributors to the total SOAFP. To improve the air quality in the study area traffic emissions as well as the aromatic and alkene VOCs emissions reduction are necessary. Unlabelled Image • Traffic had the major influence on the VOCs and other trace gases concentrations in Nanjing. • Aromatic and alkene VOCs contributed the most to the O 3 and SOA formation. • The study area was nitrogen oxides sensitive to ozone formation. [ABSTRACT FROM AUTHOR]

Secondary inorganic aerosols, including sulfate, nitrate, and ammonium (SNA), are the predominant components of fine particles (PM 2.5). Reasonable representations of SNA formation in numerical models can largely improve the predictions of PM 2.5 concentrations and effectively help implement emission control strategies. Despite the Atmospheric Pollution Prevention and Control Action Plan has been implemented since 2013, PM 2.5 concentration during 2017 in Nanjing, one of the megacities in China, still exceeded the World Health Organization-recommended safe level (35 μg m−3). In this study, WRF-Chem model was applied to simulate aerosol chemical components in PM 2.5 during April 2016 and January 2017 in Nanjing, and the simulations are evaluated with in-situ observations. Our results show that the model can reasonably reproduce the temporal variability of PM 2.5 in two seasons, but significantly underestimate the sulfate concentrations by 71% (84%) in April (January), and overestimate the nitrate concentrations by 67% (45%) in April (January). The simulated ammonium is overall consistent with the observations. Meanwhile, the model tends to overestimate SO 2 concentrations by 20% (74%) in April (January). Several sensitivity studies are conducted to explore the mechanisms for underestimation of sulfate and overestimation of nitrate, and found that the conversion rate of SO 2 to sulfate is significantly underestimated in the model. Tripling the gas-phase oxidation rate of SO 2 by OH only enhances sulfate by 67% (72%) in April (January), indicating gas-phase oxidation is not the main causes for the underestimations in the model. However, inclusion of SO 2 heterogeneous oxidation in aerosol water can largely increase the simulated sulfate by 84% (196%) in April (January), and also better reproduce the diurnal variations of sulfate compared to the reference run. It should be noted that the simulated sulfate is still 47% (53%) lower than the observations in April (January), though inclusion of heterogeneous reaction can substantially improve the simulation performance of SNA. • SO 4 underestimates by 71% (84%), while NO 3 overestimates by 45% (67%) in Apr. (Jan.). NH 4 is consistent with the observation. • Underestimated SO 4 is accompanied by overestimated SO 2 by 27% (74%) in Apr. (Jan.), indicating SO 2 oxidation rate may be low. • Compared to the reference run, tripling SO 2 gas-phase oxidation rate only increases SO 4 by 67% (72%) in Apr. (Jan.). • Incorporating SO 2 heterogeneous reaction in aerosol water can increase SO 4 y 84% (196%) in Apr. (Jan.). • Incorporating SO 2 heterogeneous reaction can also better reproduce the observed diurnal variation. [ABSTRACT FROM AUTHOR]